Price: Expect about $800/set + tires (and up!), though you can find some good deals on used sets
A lot of misguided people like to "upgrade" their car with larger-diameter wheels. There are only two situations when this is legit: if you need larger wheels to fit larger brakes (which is almost never necessary), or if you're trying to hit a sweet spot in terms of tire offerings, which aren't consistent across all sizes. Go for larger wheels for any other reason and you're just slowing your car down.
Now, lighter wheels? That's a fantastic idea. The faster you're going, the more energy that's required to turn your car, and your suspension isn't there to worry about your wheels -- or your brakes, for that matter. Any mass you can save in your wheels will pay huge dividends when you're cornering.
2. Getting a convex mirror
Price: Starting at $20
Have you ever noticed how most tractor-trailers and plenty of larger pickups have an extra mirror added on, and that it's convex? That helps the driver see a wider angle, thus minimizing the blind spot. If you have trouble with blind spots, and you've already adjusted your mirrors properly, a rearview convex mirror will be a tremendous help, especially when confronted with the sudden need to swerve and avoid a random tire/duckling/squirrel in the road.
3. Refreshing everything that pivots
Price: Under $100/pair (for either tie rods or ball joints), plus the cost of an alignment
Basically, anything that pivots will eventually wear out on your suspension and need replacement -- even if you don't know what they do, you've likely heard of ball joints and tie rod ends. If the wear is extremely bad, they actually become loose, and you do not want play in your steering or suspension. You'll recognize it in the form of a highly disconcerting wobble in the steering wheel, or a wildly uneven wear pattern in the tires. If the wear is more like that milk you didn't realize was expired until halfway through breakfast, you might not notice the looseness, but you'll definitely notice how good your car feels once everything has been replaced.
4. Getting rid of your outdated fan
Price: $75-$300 depending on how fancy or effective you need it to be. (Pro tip: you can often find a worthwhile fan in a junkyard if you do your research and know what will fit your car.)
If your car is from the '90s or earlier, your cooling fan isn't terribly efficient. It's driven by the engine, so it robs power, even when it's not needed -- something known as "parasitic power loss." It operates using a clutch that tends to wear out over time, at which point your fan's cooling ability fades dramatically. Switching to an electric fan gets back the horsepower that's otherwise lost, and keeps the fan running as long as you need to keep temperatures in check, even if your car is off.
If your car is more modern, this likely won't apply to you. It's more of an old hot rodder's trick that the manufacturers eventually adopted.
5. Changing your car's oil -- and not just in the engine
Price: Varies according to type of oil and your car's capacity, but as a general rule expect around $10/quart for the good stuff
Oil does a hell of a lot more than lubricate your engine. It helps keep it cool and prevents crucial components from sending shards of metal straight through your wallet. You absolutely want to make sure your oil is at just the right viscosity (i.e., thickness), depending on what you're using the car for. (I.e., hauling a trailer across Death Valley in the summer is a far cry from running to the grocery store and back in February, eh?)
It's more than just engine oil, though. Your transmission and your differential both have oil in them, and depending on the age of your vehicle, make sure you change the oil inside to keep everything running smoothly.
6. Installing a larger radiator
Price: $150 and up
Sexy, right? If you've ever floored your car on a hot August afternoon (under totally safe circumstances, of course) and thought Huh, weird, it usually has more power than that, then you might have heat soak. It means your car's cooling system can't get rid of the heat fast enough, and the temperature of your engine, and all of its components, rises. So long as it doesn't get extreme, the biggest drawback is a loss of power. But it's still a sign that you need more cooling capacity. A larger radiator will help here.
7. Adding brake ducts
Price: Under $100 if you make 'em yourself using high-temperature ducting and a little ingenuity
You push your foot down on the brake pedal, but you're met with very little resistance. Your body floods with adrenaline at the realization that your brake fluid has boiled and you're not slowing down remotely as much as you need to. Your brakes are too hot; good luck!
A healthy fear of overheating your braking system is understandable, given its critical importance in keeping you alive. But a lot of people go overboard and spend thousands upon thousands of dollars on fancy setups that look blingtastic, but just make the car slower from the added weight. A better set of pads, good brake fluid, and a duct that routes air directly to the center of the brake rotor is more than enough to keep high temps from being an issue.